1. Field of the Invention
The present invention relates to an optical head apparatus for use with a data processor which optically records, reproduces and erases data. More particularly, the invention relates to an optical head apparatus using a holographic element.
2. Description of the Prior Art
Like the one shown in Japanese Patent Application Laid-Open No. 56-57013 (1981), an optical head apparatus mounting with a holographic beam splitter is well known. Referring now to FIGS. 1 through 4, a conventional optical head apparatus is described below. The reference numeral 1 represents the semiconductor laser which constitutes the light source, 2 represents the light beam emitted from the semiconductor laser 1, and 8 represents the diffraction grid which is substantially a three-light-beam generating means splitting the emitted light beam 2 into three light beam each having the different condensing position. The reference numeral 3 represents the condensing lens which constitutes a means for condensing the emitted light beam 2 onto an optical disc 4 which constitutes the data memory medium. The optical disc 4 has tracks 9 for storing data on the concentricity thereof. The reference numeral 5 represents a holographic beam splitter which constitutes beam splitting means. In order to give astigmatism to a reflected light beam 6a which is the primarily diffracted light, the holographic beam splitter 5 has the stripe configuration which causes the grid cycle to gradually vary in the aperture. The holographic beam splitter 5 splits the reflective light beam 6 diffused and reflected from the optical disc 4 from the emitted light beam 2 and then converts it into the reflected light beam 6a containing the astigmatism. The reference numeral 7 represents the photodetector which receives the reflected light beam 6a. As shown in FIG. 2, the interior of the photodetector 7 is divided into four parts consisting of photoelements 7a through 7d, where the photodetector 7 is composed of a main detecting unit 7t detecting a main reflected light beam 6t, photoelements 7e and 7f provided on both sides of the main detecting unit 7t, and the arithmetic operating units 18 and 19 executing a variety of arithmetic operations based on the signals output from the photoelements 7a through 7f.
Next, functional operation of the above-cited optical head apparatus in connection with the reproduction, tracking servo, and the focus servo, is described below. The light beam 2 emitted from the semiconductor laser 1 is split into three emitted light beams 2 by the diffraction grid 8. Next, these three light beams 2 permeate the holographic beam splitter 5. Only the zero-dimensional diffracted light is radiated by a condensing lens 3 against the tracks 9 on the optical disc 4 in the state of substantially non-astigmation three condensed spot beams 10a, 10e and 10f. The reflected light beam 6 diffused and reflected from the optical disc 4 is then led into the holographic beam splitter 5 through the condensing lens 3 again, then varying its traveling direction. The traveling direction of the reflected light beam 6 from the optical disc 4 is initially diffracted by the holographic beam splitter 5 by angle .theta., and then, the reflected light beam 6a is received by the photodetector 7 which is disposed being apart from the semiconductor laser 1 by a space .delta.. The reflected light beam 6a is composed of three reflected light beams 6t, 6e and 6f when entering into the photodetector 7, 6t is received by the photoelement 7t, 6e by the photoelement 7e, and 6f by the photoelement 7f, respectively. The photoelements 7a through 7d of the main photodetecting unit 7t, detect the reflected light beam 6t respectively and then output signals. Based on these output signals, arithmetic operating units 18a through 18c, calculate "7a+7b+7c+7d" respectively, as a result, a reproduction signal 17 is generated to read the data of tracks 9.
Normally, due to occurrence of error in the installation process, the rotation axis of a driving gear of the optical-disc drive unit and the axis of the optical disc 4 do not precisely match each other. As a result, the track deviates in agreement with the rotation. The "twin-spot" method disclosed by the preceding Japanese Patent Publication No. 53-13123 (1978) for example is well known for detecting the deviated track. The method of detecting the deviated track by applying the "twin-spot" method is described below. FIG. 3 represents the ideal position relationship between the track 9 and the condensed spot beam 10 on the optical disc 4. Since the data-reading is executed by the condensed spot beam 10a, it is necessary for the system to correctly radiate the condensed spot beam 10a onto the track 9. To achieve this, the condensed spot beams 10e and 10f are condensed at the symmetrical positions centering the condensed spot beam 10a across the track 9. The condensed spot beams 10e and 10f diffuse and reflect before becoming the reflected light beams 6e and 6f respectively, which are then received by the photoelements 7e and 7f via the condensing lens 3 and the holographic beam splitter 5. The arithmetic operating element 19a calculates the signals output from the photoelements 7e and 7f, i.e., the difference of the reflection intensity between the condensed spot beams 10e and 10f before generating the differential output, and finally, a tracking error signal 11 proportional to the deviation of the track is generated. By feeding the tracking error signal 11 to a tracking actuator (not shown) which moves the condensing lens 3 in the direction perpendicular to the track 9, the optical head apparatus can constantly condense the condensed spot beams 10a at the center of the track 9.
The surface of the optical disc 4 normally is not flat, and the surface oscillates itself while the disc rotates, thus generating the focus deviation. To detect the deviated focus, as well known in Japanese Patent Publication No. 53-39123 (1978) for example, it has been proposed to give the astigmatism to the reflected light beam 6 from the optical disc 4 so as to detect the deviated focus by variation of the configuration of the light beam 6. The method of detecting the deviated focus is described below. First, the holographic beam splitter 5 gives the astigmatism to the reflected light beam 6. According to the direction of the far-and-near deviation of the focus position of the condensing lens 3 from the optical disc 4, the spot-beam configuration of the reflected light beam 6t in the detection region of the main photodetecting unit 7t of the photodetector 7 varies from the circle to the ellipse which extends in the direction 90.degree. away. When the optical disc 4 approaches the focal position, the spot-beam configuration varies from the original state shown in FIG. 4-b to the elliptic shape shown in FIG. 4-a. The spot-beam configuration also varies into the elliptic shape shown in FIG. 4-c when the optical disc 4 leaves the focal position. Photoelements 7a through 7d detect the variation of the reflected light beam 6t respectively. These photoelements 7a through 7d output signals corresponding to the volume of the received light respectively. The arithmetic operating elements 18a, 18b and 19b execute the calculation of the expression (7a+7c)-(7b+7d) to generate the comparative output, i.e., the focusing error signal 12. The deviated focus of the condensed spot beam 10 on the optical disc 4 can be corrected by operating the focusing actuator (not shown) after feeding the focusing error signal 12 to the actuator which moves the condensing lens 3 in the direction of the light axis.
The conventional optical head apparatus has such structure as described above, and thus, in order to employ the "twin-spot" method, it is necessary to largely vary the traveling direction of the reflected light beam 6a by disposing the diffraction grid 8 between the semiconductor laser 1 and the holographic beam splitter 5 so that the reflected light beam 6a cannot be shut off by the diffraction grid 8. In order to maintain the diffraction angle .theta., assume that the grid cycle of the holographic beam splitter 5 is P and the wave length of the semiconductor laser 1 is .lambda., the grid cycle given by the expression P .lambda./.theta. is required. For example, if .theta.=0.7 rad (about 40.degree.) and .lambda.=0.78 micron, then P=1.1 microns or, the extremely small grid cycle is generated. This makes it quite difficult for manufacturers to produce the holographic beam splitter 5. A longer distance .delta. is required between the semiconductor laser 1 and the photo-detector 7, as a result, there is a disadvantage that the dimension of the optical head apparatus expands.
Those disadvantages mentioned above are produced because the reflected light beam 6a must arrive at the photodetector 7 without coming into contact with the diffraction grid 8. Thus, as shown in FIG. 5, it is proposed to provide such an optical head apparatus which allows the reflected light beam 6a to arrive at the photodetector 7 after the reflected light beam 6a again permeates the diffraction grid 8. Since the diffraction angle .theta. of the holographic beam splitter 5 can be diminished, manufacturers can easily produce the above optical head apparatus. Furthermore, a more compact apparatus can be obtained because of the shorter distance .delta.. However, the permeation of the reflected light beam 6a through the diffraction grid 8 leads to the following problems.
The diffraction grid 8 splits the light beam 2 emitted from the semiconductor laser 1 into three light beams, which are diffused and reflected from the optical disc 4. The reflected light beam 6a (being composed of 6t, 6e and 6f), diffracted by the holographic beam splitter 5, again permeates the beam-splitting diffraction grid 8. Concurrently, these reflected light beams 6t, 6e and 6f are split into three light beams respectively. As a result, the photo-detector 7 detects a total of nine light beams. These light beams are represented by 6t.alpha., 6e.alpha. and 6f.alpha. (.alpha.: diffraction dimensional number .sub.-1, .sub.0 and .sub.+1) respectively. The state of the reflected light beam 6a radiating the photodetector 7 is shown in FIG. 6. In addition to the originally reflected light beam 6t, the main photodetector 7t also receives two unnecessary reflected light beams 6e.sub.+1 and 6f.sub.-1. These two reflected light beams 6e.sub.+1 and 6f.sub.-1 have already read the data from the track 9 on the optical disc 4 being different from the track 9 which have received the radiation of the condensed spot beam 10a, and as a result, those unnecessary reflected light beams make up noise component in the original signal, thus degrading the performance characteristic of the reproduced signal 17.
The tracking signal 11 is generated by the differential signal output from the photoelements 7e and 7f. When the optical disc 4 inclines itself by the surface oscillation and the like, the balance between these photoelements 7e and 7f is lost, thus causing the tracking error signal 11 to be offset. The reflected light beams 6e.sub.0 and 6t.sub.-1 superimpose themselves in the photoelement 7e. However, since the length of the light path until each light beam from the semiconductor laser 1 reaches the photoelement 7e after the reflection from the optical disc 4 is almost equal to each other, interference occurs on the photoelement 7e, as a result, the output signal does not make up the sum of the intensity of the two reflected light beams 6e.sub.0 and 6t.sub.-1.
When the optical disc 4 inclines itself, the difference of the length of the light path slightly varies to alter the state of the interference, thus, the output detection signal varies. Consequently, the tracking error signal 11 unstably varies itself.